Electron field emitter and compositions related thereto

ABSTRACT

This invention provides compositions of matter that contain an electron emitting substance and an expansion material. The expansion material may, for example, be an intercalation compound. When a film is formed from the composition, expansion of the expansion material typically causes rupturing or fracturing of the film. No further treatment of the surface of the film is typically required after expansion of the expansion material to obtain good emission properties. A surface formed from such a fractured film acts as an efficient electron field emitter and thus is useful in vacuum microelectronic devices.

This application claims the benefit of U.S. Provisional Application No.60/375,206, filed Apr. 24, 2002, which is incorporated in its entiretyas a part hereof for all purposes.

FIELD OF THE INVENTION

This invention relates to compositions of matter that are efficientelectron field emitters. In particular the invention relates to electronfield emitters that contain an electron emitting substance and anexpansion material. Further the invention relates to a process of makingan electron field emitter by expanding an expansion material.

BACKGROUND OF THE INVENTION

Field emission electron sources, often referred to as field emissionmaterials or field emitters, can be used in a variety of electronicapplications, e.g., vacuum electronic devices, flat panel computer andtelevision displays, emission gate amplifiers, and klystrons and inlighting.

Display screens are used in a wide variety of applications such as homeand commercial televisions, laptop and desktop computers and indoor andoutdoor advertising and information presentations. Flat panel displayscan be an inch or less in thickness in contrast to the deep cathode raytube monitors found on most televisions and desktop computers. Flatpanel displays are a necessity for laptop computers, but also provideadvantages in weight and size for many of the other applications.Currently laptop computer flat panel displays use liquid crystals, whichcan be switched from a transparent state to an opaque state by theapplication of small electrical signals. It is difficult to reliablyproduce these displays in sizes larger than that suitable for a laptopcomputer.

Plasma displays have been proposed as an alternative to liquid crystaldisplays. A plasma display uses tiny pixel cells of electrically chargedgases to produce an image, and its operation requires a relatively largeamount of electrical power.

Flat panel displays having a cathode that uses a field emission electronsource, i.e., a field emission material or field emitter, and a phosphorcapable of emitting light upon bombardment by electrons emitted by thefield emitter, have been proposed. Such displays have the potential forproviding the visual display advantages of the conventional cathode raytube and the depth, weight and power consumption advantages of the otherflat panel displays. U.S. Pat. Nos. 4,857,799 and 5,015,912 disclosematrix-addressed flat panel displays using micro-tip cathodesconstructed of tungsten, molybdenum or silicon. WO 94/15352, WO 94/15350and WO 94/28571 disclose flat panel displays wherein the cathodes haverelatively flat emission surfaces.

Field emission has been observed in two kinds of nanotube carbonstructures. L. A. Chernozatonskii et al [Chem. Phys. Letters 233, 63(1995) and Mat. Res. Soc. Symp. Proc. Vol. 359, 99 (1995)] have producedfilms of nanotube carbon structures on various substrates by theelectron evaporation of graphite in 10⁻⁵˜10⁻⁶ torr. These films consistof aligned tube-like carbon molecules standing next to one another. Twotypes of tube-like molecules are formed: (1) the A-tubelites, whosestructure includes single-layer graphite-like tubules formingfilaments-bundles 10–30 nm in diameter; and (2) the B-tubelites,including mostly multilayer graphite-like tubes 10–30 nm in diameterwith conoid or dome-like caps. The authors report considerable fieldelectron emission from the surface of these structures and attribute itto the high concentration of the field at the nanodimensional tips.

B. H. Fishbine et al [Mat. Res. Soc. Symp. Proc. Vol. 359, 93 (1995)]discuss experiments and theory directed towards the development of abuckytube (i.e., a carbon nanotube) cold field emitter array cathode. A.G. Rinzler et al [Science 269, 1550 (1995)] report the field emissionfrom carbon nanotubes is enhanced when the nanotubes tips are opened bylaser evaporation or oxidative etching.

W. B. Choi et al [Appl. Phys. Lett. 75, 3129 (1999)] and D. S. Chung etal [J. Vac. Sci. Technol. B 18(2)] report the fabrication of a 4.5 inchflat panel field display using single-wall carbon nanotubes-organicbinders. The single-wall carbon nanotubes were vertically aligned bysqueezing paste through a metal mesh, by surface rubbing and/or byconditioning by electric field. The authors also prepared multi-wallcarbon nanotube displays. It was noted that carbon nanotube fieldemitters having good uniformity were developed using a slurry squeezingand surface rubbing technique. Further, it was found that removing metalpowder from the uppermost surface of the emitter and aligning the carbonnanotubes by surface treatment enhanced the emission. Single-wall carbonnanotubes were found to have better emission properties than multi-wallcarbon nanotubes, but single-wall carbon nanotube films showed lessemission stability than multi-wall carbon nanotube films.

Zettl et al (U.S. Pat. No. 6,057,637) disclose a field emitter materialcomprising a volume of binder and a volume of B_(x)C_(y)N_(z) nanotubessuspended in the binder, where x, y and z indicate the relative ratiosof boron, carbon and nitrogen.

WO 01/99146 discloses a method of improving the field emission of anelectron field emitter that may be made from an acicular emittingsubstance.

N. M. Rodriguez et al [J. Catal. 144, 93 (1993)] and N. M. Rodriguez [J.Mater. Res. 8, 3233 (1993)] discuss the growth and properties of carbonfibers produced by the catalytic decomposition of certain hydrocarbonson small metal particles. U.S. Pat. Nos. 5,149,584, U.S. 5,413,866, U.S.5,458,784, U.S. 5,618,875 and U.S. 5,653,951 disclose uses for suchfibers.

Despite disclosures in the art such as those discussed above, there is acontinuing need for technology enabling the commercial use of electronemitting substances, particularly acicular carbon, in electron fieldemitters.

SUMMARY OF THE INVENTION

This invention provides compositions of matter that contain an electronemitting substance and an expansion material. The expansion materialmay, for example, be an intercalation compound. When a film is formedfrom the composition, expansion of the expansion material typicallycauses rupturing or fracturing of the film. After expansion of theexpansion material, no further treatment of the surface of the film istypically required to render the film useful as a field emitter. Aconductor formed from such a fractured film acts as an efficientelectron field emitter and thus is useful in vacuum microelectronicdevices. Other embodiments of the invention are consequently an electronemitting film that has been ruptured or fractured, and a process forfabricating an electron field emitter by expanding an expansion materialin a film from which the field emitter has been prepared.

In a preferred embodiment, this invention provides compositions ofmatter for field emission that include acicular carbon and an expansionmaterial, wherein the expansion material expands during heat treatmentand provides sufficient force to rupture or rearrange an electronemitting film made from the composition.

Carbon nanotubes are the preferred acicular carbon. Single wall carbonnanotubes are more preferred, and single wall carbon nanotubes grown bylaser ablation or irradiation are especially preferred. Preferred foruse in this process is an electron field emitter prepared from acomposition in which the electron emitting substance is about 0.1 toabout 20 wt % of the total weight of the composition, and,alternatively, may be less than about 9 wt % of the total weight of thecomposition. More preferred is an electron field emitter prepared from acomposition in which the electron emitting substance is less than about5 wt % of the total weight of the composition. Still more preferred isan electron field emitter prepared from a composition in which theelectron emitting substance is less than about 1 wt % of the totalweight of the composition. Most preferred is an electron field emitterprepared from a composition in which the electron emitting substance isabout 0.01 wt % to about 2 wt % of the total weight of the composition.

The preferred expansion material is graphite particles which areintercalated and expand in volume when heated. Other intercalationcompounds such as clays and micas may also serve this role. Expansionmaterials, such as the graphite particles, are added from 1 wt % to 99wt % of the total solids weight of the composition containing anelectron emitting substance. Other materials such as silver particles,glass particles and organic vehicles may be added to the formulation inorder to add printability, conductivity or insulation to thecomposition.

The composition may be prepared as a screen printable paste, containingamong the solids an electron emitting substance such as carbonnanotubes, wherein the electron emitting substance is about 0.1 to about20 wt % of the total weight of the solids in the paste, but may be lessthan 9 wt % of the total weight of the solids in the paste. Morepreferred is a composition wherein the electron emitting substance isless than 5 wt % of the total weight of the solids in the paste. Stillmore preferred is a composition wherein the electron emitting substanceis less than 1 wt % of the total weight of the solids in the paste. Mostpreferred is a composition wherein the electron emitting substance isabout 0.01 wt % to about 2 wt % of the total weight of the solids in thepaste. This paste is especially useful in fabricating an electron fieldemitter by the process of the invention. Such an emitter has excellentemission properties, good adhesion to a substrate along with theadvantages of ease of preparing and comparatively low cost of materialsand processing.

The improved electron field emitters of this invention are fabricatedfrom the compositions of this invention, and are useful in flat panelcomputer, television and other types of displays, vacuum electronicdevices, emission gate amplifiers, klystrons and in lighting devices.The compositions of matter and process hereof are especiallyadvantageous for producing large area electron field emitters for flatpanel displays, i.e. for displays greater than 30 inches (76 cm) insize. The flat panel displays can be planar or curved.

Another embodiment of this invention is a composition of mattercontaining (a) an electron emitting substance, and (b) an expansionmaterial, the volume of which is expandable by a factor of at leastabout 1.03 times.

A further embodiment of this invention is a composition of matter (a) anelectron emitting substance, and (b) an intercalation compound.

Yet another embodiment of this invention is an electron emitting filmcontaining (a) an electron emitting substance, and (b) an expansionmaterial, wherein the electron emitting film has been ruptured byexpansion of the expansion material.

Yet another embodiment of this invention is a process for fabricating anelectron emitting film by (a) forming an electron emitting film from acomposition that contains (i) an electron emitting substance, and (ii)an expansion material, the volume of which is expandable by a factor ofat least about 1.03 times; and (b) expanding the expansion material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the surface microstructure of a composition of thisinvention before and after heat treatment.

FIG. 2 shows the I–V curve after a film fabricated from a composition ofthis invention (Example 1) has been heat treated.

FIG. 3 shows the I–V curve after a film fabricated from a compositionnot described by this invention (Control A) has been heat treated.

FIG. 4 shows a comparison of the I–V curves of Example 1 and Control A.

FIG. 5 shows the I–V curve after a film fabricated from a composition ofthis invention (Example 2) has been heat treated.

DETAILED DESCRIPTION OF THE INVENTION

This invention provides a composition of matter with improved fieldemission that contain an electron emitting substance and an expansionmaterial. The composition may be used for the fabrication of an electronfield emitter. This composition may contain, in addition to an electronemitting substance and an expansion material, glass frits, metallicpowder or metallic paint, or a mixture thereof, as may be desired forassistance in attachment of the electron field emitter to a substrate.As a result, the total weight of the composition from which the electronfield emitter may be fabricated does include the weight of materialssuch as glass frits, metallic powder or metallic paint, but does notinclude the weight of the substrate that supports the electron fieldemitter.

The composition herein is especially effective when the electronemitting substance is an acicular emitting substance, e.g., carbon, asemiconductor, metal or mixtures thereof. An acicular substance iselongated and contains particles with aspect ratios of 10 or more.Acicular carbon can be of various types. Carbon nanotubes are thepreferred acicular carbon, and single wall carbon nanotubes areespecially preferred. The individual single wall carbon nanotubes areextremely small, typically about 1.5 nm in diameter. The carbonnanotubes are sometimes described as graphite-like, presumably becauseof the sp² hybridized carbon. The wall of a single wall carbon nanotubecan be envisioned as a cylinder formed by rolling up a graphene sheet.Multiwall nanotubes have cylindrical walls of more than one graphenesheet, and may also be used in the invention.

Carbon fibers grown from the catalytic decomposition ofcarbon-containing gases over small metal particles are also useful asthe acicular carbon. A catalytically grown carbon fiber is a carbonfiber grown from the catalytic decomposition of carbon-containing gasesover small metal particles, and has graphene platelets arranged at anangle with respect to the fiber axis so that the periphery of the carbonfiber consists essentially of the edges of the graphene platelets. Theangle may be 90°, or may be an acute angle with respect to theperpendicular to the fiber axis.

Other examples of acicular carbon are polyacrylonitrile-based(PAN-based) carbon fibers and pitch-based carbon fibers.

The expansion material may be an intercalation compound such asintercalated graphite, mica, clay or vermiculite. An intercalationcompound is a chemical compound in which a crystalline substanceincorporates molecules, atoms or ions of another substance in gaps orlayers of its crystal lattice. The crystalline lattice acts as anelectron donor, and “foreign” electron acceptor atoms are interspersedor diffused between the planes of the lattice. Graphite is particularlysusceptible to this phenomenon because of its orderly stacked layers ofcrystals. The volume of the expansion material is expandable by a factorof about 1.03 to about 200 times, and preferably the volume of theexpansion material is expandable by a factor of at least about 1.03times. Usually this expansion in volume is caused by a treatment such asheat treating. In an alternative embodiment, the expansion material maybe a foamable material. The expansion material may be used in acomposition of this invention in an amount of about 1 wt % to about 99wt %, based on the total weight of the composition, preferably in anamount of about 2 wt % to about 30 wt %, and more preferably in anamount of about 5 wt % to about 20 wt %.

When the composition of this invention is used to prepare an electronfield emitter, various processes can be used to attach the compositionto a substrate. As the electron field emitter thus prepared will beincorporated into an apparatus such as a field emitter cathode, theattachment between the composition and the substrate must withstand andmaintain its integrity under the conditions of manufacturing and theconditions of usage of such apparatus. Those conditions are typicallyvacuum conditions and temperatures up to about 450° C. As a result,organic materials are not generally applicable for attaching an electronfield emitter to a substrate, and the poor adhesion of many inorganicmaterials to carbon further limits the choice of materials that can beused. A preferred method of attachment is to screen print thecomposition in the form of a paste, optionally containing glass frits,metallic powder or metallic paint or a mixture thereof, onto a substratein the desired pattern and to then fire the dried patterned paste. For awider variety of applications, e.g., those requiring finer resolution,the preferred process involves screen printing a paste that contains aphotoinitiator and a photohardenable monomer, photopatterning the driedpaste and firing the patterned paste.

The substrate can be any material to which a paste composition willadhere. If the paste is non-conducting and a non-conducting substrate isused, a film of an electrical conductor to serve as the cathodeelectrode, and to provide means to apply a voltage to and supplyelectrons to the electron emitting substance, will be needed. Silicon, aglass, a metal or a refractory material such as alumina can serve as thesubstrate. For display applications, the preferable substrate is glass,and soda lime glass is especially preferred. For optimum conductivity onglass, silver paste can be pre-fired onto the glass at 500–550° C. inair or nitrogen, but preferably in air. The conducting layer so-formedcan then be over-printed with the composition in paste form.

The paste used for screen printing contains an expansion material, anelectron emitting substance, such as acicular carbon. It frequently alsocontains an organic medium, a solvent, a surfactant, alow-softening-point glass frit, a metallic powder and/or a metallicpaint, or a mixture of any of same. The role of the organic medium andsolvent is to suspend and disperse the particulate constituents, i.e.the solids, in the paste with a proper rheology for typical patterningprocesses such as screen printing. There are many such organic mediaknown in the art. Examples of resins that can be used for such purposeare cellulosic resins such as ethyl cellulose and alkyd resins ofvarious molecular weights. Butyl carbitol, butyl carbitol acetate,dibutyl carbitol, dibutyl phthalate and terpineol are examples of usefulsolvents. These and other solvents are formulated to obtain the desiredviscosity and volatility requirements in the composition. A surfactantcan be used to improve the dispersion of the particles. Organic acidssuch oleic and stearic acids and organic phosphates such as lecithin orGafac® phosphates are typical surfactants.

A glass frit that softens sufficiently at the firing temperature toadhere to the substrate and to the electron emitting substance istypically used. Lead or bismuth glass frits can be used, as well asother glasses with low softening points, such as calcium or zincborosilicates. Within this class of glasses, the specific composition isgenerally not critical. If a screen printable composition with higherelectrical conductivity is desired, the paste typically also contains aconductive metal, for example, silver or gold. The paste typicallycontains about 40 wt % to about 80 wt % solids based on the total weightof the paste. These solids comprise the electron emitting substance,together with glass frits and/or metallic components, as desired.Variations in the composition can be used to adjust the viscosity andthe final thickness of the printed material.

The emitter paste is typically prepared by three-roll milling a mixtureof the electron emitting substance, the expansion material, and, asneeded, an organic medium, a surfactant, a solvent, alow-softening-point glass frit, a metallic powder, and/or a metallicpaint, or a mixture thereof. The paste mixture can be screen printedusing well-known screen printing techniques, e.g. by using a165–400-mesh stainless steel screen. The paste can be deposited as acontinuous film or in the form of a desired pattern. When the substrateis glass, the paste is then fired at a temperature of about 350° C. toabout 550° C., preferably at about 450° C. to about 525° C., for about10 minutes in nitrogen. Higher firing temperatures can be used withsubstrates which can endure them provided the atmosphere is free ofoxygen. However, the organic constituents in the paste are effectivelyvolatilized at 350–450° C., leaving the layer of composite comprised ofthe electron emitting substance and other components such as glass fritand/or metallic conductor.

If the screen-printed paste is to be photopatterned, the paste maycontain a photoinitiator, a developable binder and/or a photohardenablemonomer, such as at least one addition polymerizable ethylenicallyunsaturated compound having at least one polymerizable ethylenic group.

A preferred composition for use as a screen printable paste is onecontaining solids that include an electron emitting substance, such ascarbon nanotubes, wherein the electron emitting substance is less than 9wt % of the total weight of solids in the paste. More preferred is acomposition wherein the electron emitting substance is less than 5 wt %of the total weight of solids in the paste. Still more preferred is acomposition wherein the electron emitting substance is less than 1 wt %of the total weight of solids in the paste. Most preferred is acomposition wherein the electron emitting substance is about 0.01 wt %to about 2 wt % of the total weight of solids in the paste.

The paste described above is especially useful in fabricating anelectron field emitter by the process of the invention. Compositionswith a low concentration of electron emitting substance provide anexcellent electron field emitter when fabricated by the process of thisinvention. As a typical example, a paste containing an electron emittingsubstance, an expansion material, glass frit and silver will containabout 0.01 to about 6.0 wt % electron emitting substance, about 5 toabout 10 wt % expansion material, about 3 to about 15 wt % glass frit,and about 40 to about 75 wt % silver in the form of fine silverparticles, based on the total weight of the paste.

The compositions of this invention are useful for the fabrication of anelectron field emitter having improved field emission properties, andthey include an electron emitting substance, such as acicular carbon, anacicular semiconductor, an acicular metal, or mixtures thereof, inaddition to an expansion material. An electron field emitter isfabricated from an electron emitting film that is in turn prepared froma composition of this invention. In the process of this invention, atreatment, such as a heat treatment, is applied to an electron emittingfilm. Upon the application of the treatment, the expansion materialexpands and produces sufficient displacement and force to rupture orrearrange a portion of the field emitting film, thereby forming a newsurface of the field emitting film. In FIG. 1, the surface of a filmbefore an after expansion of an expansion material is shown. The surfacebefore expansion is shown in FIG. 1 a (50× magnification), and thesurface after expansion is shown in FIGS. 1 b (500× magnification) and 1c (3000× magnification). FIGS. 1 b and 1 c show ruptures in the surfaceof the film. In this ruptured or fractured condition, the field emittingfilm has improved field emission performance as compared to a film thathas not been ruptured or fractured. It is believed that the newly formedsurface of the field emitting film has electron emitting particles, suchas acicular particles, protruding from it.

Electron field emitters will have improved emission properties by virtueof incorporating a film that has been subjected to the process of thisinvention. The process of this invention is thus not only a process forfabricating an electron emitting film, but is also a process forfabricating an electron field emitter because the film may be includedin the field emitter. The improved electron field emitters of thisinvention can be used in the cathodes of electronic devices such asfield emission triodes and in particular in field emission displaydevices. Such a display device contains (a) a cathode using an electronfield emitter improved by the process of this invention, (b) a patternedoptically transparent electrically conductive film serving as an anodeand spaced apart from the cathode, (c) a phosphor layer capable ofemitting light upon bombardment by electrons emitted by the electronfield emitter and positioned adjacent to the anode and between the anodeand the cathode, and (d) one or more gate electrodes disposed betweenthe phosphor layer and the cathode. The use of the compositions of thisinvention to fabricate an electron field emitter is readily adapted tolarge size electron field emitters that can be used in the cathodes oflarge size display panels.

Use of the compositions of this invention for fabricating an electronfield emitter is conducive to fabricating completely screen-printedtriodes. Expansion of the expanding material in the electron emittingfilm of the electron field emitter can occur immediately after it isscreen printed and fired or, preferably, after any dielectric materialsand gate electrodes have been screen printed onto the cathode and fired.

The accuracy and resolution attained with screen printing are limited.Therefore, it is difficult to fabricate a triode with dimensions lessthan 100 μm. Preventing electrical shorting between the gate and emitterlayers is difficult due to printing inaccuracy. In addition, since thefeatures on each layer must be printed one layer at a time, repeatedrepositioning of different screens further degrades registration. Inorder to prevent shorting, the gate layer opening is often enlargedrelative to the dielectric via and this significantly degrades theeffectiveness of the gate-switching field due to increased gate toemitter distance.

A photoimagable thick film approach can solve all of the aforementionedproblems and is useful for forming an array of normal gate triodes aswell as for forming an array of inverted-gate triodes. A normal gatetriode has the gate electrode physically between the field emittercathode and the anode. An inverted-gate triode has the field emittercathode physically between the gate and the anode. Photoimagable thickfilm formulations such as the Fodel® silver and dielectric pastecompositions (DC206 and DG201 respectively) are available from E. I. duPont de Nemours and Company, Wilmington, Del. They contain silver ordielectric in the form of fine particles and a small amount of lowmelting glass frit in an organic medium containing photoimagableingredients such as photoinitiator and photomonomers. Typically, auniform layer of Fodel® paste is screen printed on a substrate withcontrolled thickness. The layer is baked in low heat to dry. A contactphoto-mask with the desired pattern is placed in intimate contact withthe film and exposed to ultra-violet (UV) radiation. The film is thendeveloped in weak aqueous sodium carbonate. Feature dimensions as smallas 10 μm can be produced by photoimaging these screen-printed thickfilms. Therefore, triode dimensions as small as 25 μm may be achieved.

In addition, imaging can be carried out in multi-layers thus eliminatingalignment accuracy problems. This is advantageous in the fabrication ofthe normal gate triode since the silver gate and dielectric layers canbe imaged together to achieve perfect alignment between the gate anddielectric openings and in the fabrication of the inverted gate triodesince the emitter, silver cathode, and dielectric layers can be imagedtogether to achieve perfect capping of the dielectric ribs whileavoiding short formation.

Use of the compositions of this invention for fabricating an electronfield emitter is also conducive to fabricating a lighting device. Such adevice comprises (a) a cathode using an electron field emitter that hasbeen fabricated by the process of the invention, and (b) an opticallytransparent electrically conductive film serving as an anode and spacedapart from the cathode, and (c) a phosphor layer capable of emittinglight upon bombardment by electrons emitted by the electron fieldemitter and positioned adjacent to the anode and between the anode andthe cathode. The cathode may consist of an electron field emitter in theform of a square, rectangle, circle, ellipse or any other desirableshape with the electron field emitter uniformly distributed within theshape or the electron field emitter may be patterned. Although screenprinting is a convenient method for forming the electron field emitter,other patterning techniques such as ink jets, stenciling or contactprinting may be used. Use of the compositions of this invention forfabricating an electron field emitter is also conducive to fabricating avacuum electronic device.

The advantageous effects of this invention are demonstrated by a seriesof examples, as described below. The embodiments of the invention onwhich the examples are based are illustrative only, and do not limit thescope of the invention. The significance of the examples (Examples 1 and2) is better understood by comparing these embodiments of the inventionwith a controlled formulation (Control A), which does not possess thedistinguishing features of this invention.

Electron field emitters were fabricated for use as samples from thecompositions of this invention and by the process of this invention.Field emission tests were carried out on the resulting samples using aflat-plate emission measurement unit comprised of two electrodes, oneserving as the anode or collector and the other serving as the cathode.The cathode consists of a copper block mounted in apolytetrafluoroethylene (PTFE) holder. The copper block is recessed in a1 inch by 1 inch (2.5 cm×2.5 cm) area of PTFE and the sample substrateis mounted to the copper block with electrical contact being madebetween the copper block and the sample substrate by means of coppertape. A high voltage lead is attached to the copper block. The anode isheld parallel to the sample at a distance, which can be varied, but oncechosen it was held fixed for a given set of measurements on a sample.Unless stated otherwise a spacing of 1.25 mm was used. The anodeconsists of a glass plate coated with indium tin oxide deposited bychemical vapor deposition. It is then coated with a standard ZnS-basedphosphor, Phosphor P-31, Type 139 obtained from Electronic SpaceProducts International. An electrode is attached to the indium tin oxidecoating.

The test apparatus is inserted into a vacuum system, and the system wasevacuated to a base pressure below 1×10⁻⁵ torr (1.3×10⁻³ Pa). A negativevoltage pulse with typical pulse width of 3 μsec at a frequency of 60 Hzis applied to the cathode and the average emission current was measuredas a function of the applied voltage.

EXAMPLE 1

This demonstrates the good emission exhibited by an electron fieldemitter fabricated from a composition of, and by the process of, thisinvention.

The emitter paste was prepared by mixing three components: one asuspension containing single wall carbon nanotubes, one a typicalorganic medium containing 10% ethylcellulose and 90% beta-terpineol, andone a typical paste containing silver. Laser ablation grown single wallcarbon nanotubes were obtained from Tubes @ Rice, Rice University,Houston, Tex. as an unpurified powder produced by laser ablation. Ananotube suspension was prepared by sonicating, i.e. by mixingultrasonically, a mixture containing 1% by weight of the nanotube powderand 99% by weight of trimethylbenzene. The ultrasonic mixer used was aDukane Model 92196 with a ¼ inch diameter horn operating at 40 kHz and20 watts. The silver paste was a silver paste composition 7095 availablefrom E. I. du Pont de Nemours and Company, Wilmington, Del. containing65.2 wt % silver in the form of fine silver particles and a small amountof glass frit in an organic medium.

Grafguard 160–150B expandable graphite flake was obtained from GraftechInc., Cleveland, Ohio, as an expansion material. 0.28 grams of Grafguardflake was heated in air at 250° C. for 10 minutes and then added to 4grams of the paste containing carbon nanotubes described above. Thecombination was mixed in a three-roll mill for ten passes to form theemitter paste. A 2 cm² square pattern of emitter paste was then screenprinted onto the pre-fired silvered glass substrate using a 200 meshscreen and the sample was subsequently dried at 125° C. for 10 minutes.The dried sample was then fired in air for 1 minute at 350° C. Afterfiring the thick film composite forms an adherent coating on thesubstrate. The expandable graphite disrupts the film in local areas.

This electron field emitter was tested for field emission as describedin the specification. FIG. 2 shows the emission results for the electronfield emitter of Example 1 with emission current plotted as a functionof applied electric field.

Control A: This demonstrates the poor emission exhibited by an electronfield emitter containing single wall carbon nanotubes combined with aparticulate material that does not expand within the film formed fromthe composition in which the carbon nanotubes are contained.

The emitter paste was prepared by mixing three components: one asuspension containing single wall carbon nanotubes, one a typicalorganic medium containing 10% ethylcellulose and 90% beta-terpineol, andone a typical paste containing silver. Laser ablation grown single wallcarbon nanotubes were obtained from Tubes @ Rice, Rice University,Houston, Tex. as an unpurified powder produced by laser ablation. Ananotube suspension was prepared by sonicating, i.e. by mixingultrasonically, a mixture containing 1% by weight of the nanotube powderand 99% by weight of trimethylbenzene. The ultrasonic mixer used was aDukane Model 92196 with a ¼ inch diameter horn operating at 40 kHz and20 watts. The silver paste was a silver paste composition 7095 availablefrom E. I. du Pont de Nemours and Company, Wilmington, Del. containing65.2 wt % silver in the form of fine silver particles and a small amountof glass frit in an organic medium.

0.28 grams of SiC platelets obtained from Third Millenium TechnologiesInc., Knoxville, Tenn. were added to 4 grams of the paste containingcarbon nanotubes described above. The particle size is very similar tothe size of the graphite particles added in Example 1. The combinationwas mixed in a three-roll mill for ten passes to form the emitter paste.A 2 cm² square pattern of emitter paste was then screen printed onto thepre-fired silvered glass substrate using a 325 mesh screen and thesample was subsequently dried at 120° C. for 10 minutes. The driedsample was then fired in nitrogen for 10 minutes at 450° C. After firingthe nanotube paste forms an adherent coating on the substrate. In thiscase there is no disruption of the nanotube containing film.

This electron field emitter was tested for field emission as describedin the specification. FIG. 3 shows the emission results for the electronfield emitter of Control A with emission current density plotted as afunction of applied electric field. FIG. 4 compares the emission resultsfrom Example 1 and Control A. Note that, as compared to Control A,Example 1 shows much higher emission current, which is indicative ofcommercial usefulness.

EXAMPLE 2

This demonstrates the good emission exhibited by an electron fieldemitter fabricated from a composition of, and by the process of, thisinvention.

The emitter paste was prepared by mixing three components: one asuspension containing single wall carbon nanotubes, one a typicalorganic medium containing 10% ethylcellulose and 90% beta-terpineol, andone a typical paste containing silver. Hipco™ process single wall carbonnanotubes were obtained from Carbon Nanotechnologies Inc., Houston, Tex.as an unpurified powder produced by catalytic decomposition of carbonmonoxide. A nanotube suspension was prepared by sonicating, i.e. bymixing ultrasonically, a mixture containing 1% by weight of the nanotubepowder and 99% by weight of trimethylbenzene. The ultrasonic mixer usedwas a Dukane Model 92196 with a ¼ inch diameter horn operating at 40 kHzand 20 watts. The silver paste was a silver paste composition 7095available from E. I. du Pont de Nemours and Company, Wilmington, Del.containing 65.2 wt % silver in the form of fine silver particles and asmall amount of glass frit in an organic medium.

Grafguard 160–150B expandable graphite flake was obtained from GraftechInc., Cleveland, Ohio. 0.28 grams of Grafguard flake was heated in airat 250° C. for 10 minutes and then added to 4 grams of the pastecontaining carbon nanotubes described above. The combination was mixedin a three-roll mill for ten passes to form the emitter paste. A 2 cm²square pattern of emitter paste was then screen printed onto thepre-fired silvered glass substrate using a 200 mesh screen and thesample was subsequently dried at 125° C. for 10 minutes. The driedsample was then fired in air for 1 minute at 350° C. After firing thethick film composite forms an adherent coating on the substrate. Theexpandable graphite disrupts the film in local areas.

This electron field emitter was tested for field emission as describedin the specification. FIG. 5 shows the emission results for the electronfield emitter with emission current plotted as a function of appliedelectric field. Note that the results are similar to Example 1, that is,good emission.

1. A composition of matter comprising (a) an acicular electron emittingsubstance as a first material, and (b) an expansion material as a secondmaterial, the volume of which is expandable by a factor of at leastabout 1.03 times, wherein the expansion material is a graphite, clay orvermiculite.
 2. A composition according to claim 1 wherein the volume ofthe expansion material is expandable by a factor of about 1.03 to about200 times.
 3. A composition according to claim 1 wherein the electronemitting substance is acicular carbon.
 4. A composition according toclaim 1 wherein the electron emitting substance is a carbon nanotube. 5.A composition according to claim 4 wherein the carbon nanotubes compriseone or both of single-wall carbon nanotubes and multi-wall carbonnanotubes.
 6. A composition according to claim 1 wherein the content ofthe electron emitting substance is about 0.1 to about 20 weight percent,based on the total weight of the composition.
 7. A composition accordingto claim 1 further comprising a conductive metal.
 8. A compositionaccording to claim 1 further comprising one or more of a photoinitiator,a developable binder and a photohardenable monomer.
 9. A compositionaccording to claim 1 in the form of a printable paste.
 10. A compositionaccording to claim 1 in the form of an electron emitting film.
 11. Anelectron emitting film comprising (a) an electron emitting substance asa first material, and (b) an expansion material as a second material,wherein the electron emitting film has been ruptured by expansion of theexpansion material, and wherein the expansion material is anintercalated graphite, clay or vermiculite.
 12. An electron emittingfilm according to claim 11 wherein the electron emitting substance iscarbon nanotube.
 13. A film according to claim 12 wherein the carbonnanotubes comprise one or both of single-wall carbon nanotubes andmulti-wall carbon nanotubes.
 14. An electron emitting film according toclaim 11 further comprising a conductive metal.
 15. An electron emittingfilm according to claim 11 further comprising one or more of aphotoinitiator, a developable binder and a photohardenable monomer. 16.A field emission triode, a field emission display, a lighting device, ora vacuum electronic device comprising an electron emitting filmaccording to claim
 11. 17. A process for fabricating an electronemitting film, comprising (a) forming an electron emitting film from acomposition that comprises (i) an electron emitting substance as a firstmaterial, and (ii) an expansion material as a second material, thevolume of which is expandable by a factor of at least about 1.03 times,wherein the expansion material is an intercalated graphite, clay orvermiculite; and (b) expanding the expansion material.
 18. A processaccording to claim 17 wherein the volume of the expansion material isexpandable by a factor of about 1.03 to about 200 times.
 19. A processaccording to claim 17 wherein the expansion material is expanded byheating.
 20. A process according to claim 17 wherein the step ofexpanding the expansion material comprises a step of rupturing the film.21. A process according to claim 17 wherein the step of forming anelectron emitting film comprises screen printing a paste prepared fromthe composition of the electron emitting substance and the expansionmaterial.
 22. A process according to claim 17 further comprising a stepof providing the electron emitting substance by growth train thecatalytical decomposition of carbon-containing gases over small metalparticles.
 23. A process according to claim 17 further comprising a stepof providing the electron emitting substance by laser irradiation.
 24. Aprocess according to claim 17 wherein the electron emitting substance isa carbon nanotube.
 25. A process according to claim 24 wherein thecarbon nanotubes comprise one or both of single-wall carbon nanotubesand multi-wall carbon nanotubes.
 26. A process according to claim 17further comprising a step of incorporating the electron field emittingfilm into the cathode of an electronic device.